This invention relates to a device for determining one or more medical variables to be measured in living organisms, comprising a part to be inserted into the tissue of the organism, which has an exchange channel with openings into the tissue, and further comprising a delivery unit connected with the exchange channel for the delivery of a perfusion fluid and its drainage after partial equilibration with the medical variable, i.e., preferably glucose concentration, and comparatively constant endogenous or exogenous marker variables, and further comprising an analyzing unit for measuring the medical variable and one marker variable and an evaluation unit determining the actual concentration of the medical variable with the use of the marker variable.